Scenario-Based Assessment and Regulatory Pathway Selection for Urban Water Metabolism Systems: A Case Study of Beijing

Regulating urban water metabolism systems requires not only comparing the effects of alternative regulatory scenarios, but also determining whether water environmental carrying-capacity constraints can be overcome while socioeconomic–water-environment coordination is maintained. Building on our previously developed and validated agent-based modeling–system dynamics (ABM–SD)-coupled model for Beijing, this study simulated a business-as-usual (BAU) scenario, a Planning scenario, and six sustainable development scenarios for 2024–2035. Total water use, COD and ammonia nitrogen emissions, the Nemerow index (PI), and coupling coordination degree (D) were evaluated. The D–PI method was extended from baseline diagnosis to multi-scenario comparison to identify regulatory pathways from their trajectories, threshold crossings, and final zones. Results show that the BAU scenario exhibits improving coordination but accumulating carrying-capacity risk. Under the Planning scenario, D increases to 0.912 and PI decreases to 1.732 by 2035, yet the system remains severely overloaded. Under SS1-2, SS2-2, and SS3-2, PI decreases to 0.881, 0.904, and 0.934, while D increases to 0.918, 0.923, and 0.927, respectively, enabling the system to enter the Coordination–Sustainability Zone. Under the recommended SS2-2 scenario, simulated total water use, COD emissions, and ammonia nitrogen emissions in 2035 are 35.49 × 108 m3, 76,038 t, and 5900 t, respectively. Among the candidate pathways, SS1-2 achieves the highest mean D and lowest mean PI over 2024–2035, SS2-2 is conditionally recommended when retaining the specified planning-oriented development settings, and SS3-2 represents a high-growth constraint test.